1. Field of Invention
This invention relates to a method and apparatus for indicating and controlling the temperature of power transformers of the type used by public utilities. This type of transformer typically has its transformer windings immersed in a liquid coolant. During operation of the power transformer it is important to know the temperature of the transformer winding, the maximum temperature it has reached, when cooling fans should be turned on and off and when power to the transformer should be turned off.
2. Discussion of Prior Art
It is known from prior systems to simulate the winding temperature by measuring the top oil temperature of the transformer and biasing the thermometer reading by an amount proportional to transformer load, or winding current, so as to indicate winding temperature. Such a system employs a thermometer with a sensing element measuring top oil temperature, a current transformer to provide a signal proportional to transformer load, a heater coil to provide the bias and an auto transformer to adjust the current supplied to the heater coil.
It is a object of this invention, rather than have a biasing heater to modulate the actual oil temperature in a transformer, to provide a method and apparatus to electronically determine the winding temperature based on the oil temperature in the transformer and the increased temperature that results from current flowing through the transformer winding.
It is a further object to provide an improved adjustable transformer temperature controller which may be adjusted or calibrated to provide a variable temperature increment to the oil temperature in accordance with the transformer manufacturers specification for temperature change at various transformer loads or winding currents.